China surged clear at the top of the Olympic medals table on Sunday, adding two more golds to their opening day haul as swimming star James Magnussen put Australia on course for relay glory.

After dominating day one of competition with four gold medals, Chinese athletes hit paydirt once again with wins in shooting for Guo Wenjun before synchronised divers Wu Minxia and He Zi triumphed at the Aquatics Centre.

Guo successfully defended her 10m air pistol title with a thrilling last-shot finish at the Royal Artillery Barracks to overhaul France's Celine Goberville, who had led going into the last round.

"I just focused on doing my best on the final shot," Guo said, adding: "I think everyone is good, I just never give up."

Legendary diver Wu meanwhile clinched her third consecutive gold medal in the 3m springboard synchronised after wins in 2004 and 2008.

But while China celebrated their burgeoning gold haul, hosts Great Britain were still waiting for their first win with cyclist Lizzie Armitstead having to settle for silver in a thrilling road race won in driving rain by Marianne Vos of the Netherlands.

United States shooter Kim Rhode meanwhile took the gold medal in the women's skeet, her fifth consecutive visit to the podium following medals at the Atlanta, Sydney, Athens and Beijing Olympics.

Elsewhere Sunday the women's gymnastics competition was rocked by a failed drugs test, as organisers launched a probe into rows of empty seats at venues.

In the pool, world champion Magnussen -- tipped to be one of the stars of the Games -- dived in at the last change to reel in US veteran Jason Lezak and lift Australia into Sunday night's relay final as top qualifiers.

It was Magnussen's first swim at the Games and the 'Missile' delivered on his brash confidence, clocking a sizzling 47.35 seconds to surge past Lezak (48.04).

Defending Olympic champions America were second quickest ahead of Russia and France.

"I felt nice and relaxed in the first 50 and I just let the crowd carry me home at the end," Magnussen said.

The Americans, who have won eight of the 10 Olympic gold medals awarded in the event, are expected to bring in big guns Nathan Adrian, Cullen Jones and Michael Phelps for Sunday night's final.

Sun Yang led the way into the men's 200m freestyle semi-finals just one day after he gave China it's first men's swimming title.

US teenager Missy Franklin, swimming in seven events, was second-quickest.

Galiulina underwent a drugs test on July 25, three days before the start of the women's artistic gymnastics competition.

Meanwhile, the chairman of the London Olympics, Sebastian Coe, insisted most venues were full of spectators as organisers launched a probe into blocks of empty seats at some events.

Coe said unoccupied seats at some venues were due to accredited officials still working out which events to attend, but he said he had visited four events on Saturday which were full to capacity.

"Let us put this in perspective. Those venues are stuffed to the gunnels. The public are in there," Coe said.

"There are tens of thousands of people at this moment within the accredited 'family' that are trying to figure out what their day looks like, where they are going to be asked to go to, frankly working out how you divide your time."

In other action on Sunday, the United States' collection of NBA multi-millionaires, who are expected to cruise to a successful defence of the basketball title, were facing France in their opening game.

Over at Wimbledon, Maria Sharapova takes on Israel's Sharar Peer as the women's tennis first round is completed.

Former world number one Novak Djokovic and home hope Andy Murray, who was knocked out of the doubles on Saturday, are also in action.

A total of 14 medals are up for grabs on Sunday, with two also on offer in judo and weightlifting, as well as one each in fencing and archery.